1. Field of the Invention
The present invention relates generally to electric motors. The present invention relates more specifically to induction motors utilized in applications demanding a range of variable speeds.
2. Discussion of the Related Art
Many applications for electric motors demand variable speeds with a known load on the motor. For example a blower motor in a household heating, ventilation and air-conditioning (HVAC) system will typically be a fractional horsepower motor driving a blower unit or fan blade which represents a known load varying regularly by speed in revolutions per minute.
Inexpensive induction motors are desirably utilized in many applications. These motors are not particularly well adapted for variable speed usage. Rather they are designed to operate efficiently only at one best speed and inefficiencies result when trying to run the motor at other than the designed speed. However, many systems, such as the above HVAC applications, would benefit greatly from having a wider range of motor speeds available.
In the past art, a variable range of speeds from one induction motor was obtained through the use of expensive controllers changing the frequency and voltage of the input to the motor windings or by using a multi-tap motor to attain a number of fixed selectable speeds by mechanical switching between the taps.
Expensive controllers such as these were necessary because, as the input to the motor windings strays farther from sinusoidal, motor efficiency and power factor drop while total harmonic distortion rises, resulting in unacceptable noise, heat, efficiency loss, and motor life.
Thus, known motor controllers utilizing inexpensive switching mechanisms, such as triacs, to control power to the motor windings by "chopping" the sinusoidal waveform input were thought to be of limited use in applications of continuously variable motor speed control.
In an article entitled "A Single Phase Induction Motor Voltage Controller with Improved Performance", J. D. Law, T. A. Lipo, IEEE Transactions on Power Electronics, Vol. PE-1, No. 4, October 1986, pp 240-247; triac control of paired main and auxiliary windings is suggested to run the pairs first in series then in parallel to maintain constant motor speed as the load varies from a low to a high, or fully rated, load. A constant firing delay angle based on empirical study is input to the triac controller using DIP switches. The phase delay is measured with a voltage zero crossing detector and zero current detector. The current hold off angle is then computed and adjusted to make the phase delay and current hold off equal to the predetermined firing delay to maintain constant rated or near-rated speed under the varying load conditions to maintain as closely as possible the full speed the motor was designed for.
The present invention is rather concerned with the opposite effect of obtaining reasonably efficient variable speed for a load of known characteristics with a low cost induction motor and controller system.